allow customize buck2 and python path (#2044)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2044

For my development flow, I'd need to have buck2 and python3 in a different path. Modify the script to allow us to customize the path

Test with
```
sh backends/qualcomm/scripts/build.sh
```

Reviewed By: kirklandsign

Differential Revision: D54078165

fbshipit-source-id: 9471b03f9f3045c450ceb17a20715da4cfe221a9